High-pressure melting and elastic behavior of vanadium and niobium based on ab initio and machine learning molecular dynamics

TL;DR

This study combines ab initio and machine learning molecular dynamics to explore high-pressure melting and elastic behaviors of vanadium and niobium, revealing phase stability, melting lines, and temperature-dependent elastic anomalies.

Contribution

It introduces a combined AIMD and MLMD approach to accurately model melting and elastic behaviors of V and Nb under high pressure, including phase stability and anomalous properties.

Findings

Nb's high-temperature Pnma phase is mechanically unstable and reverts to BCC.

The melting temperature of Nb is higher than previous estimates.

Atomic thermal displacements influence elastic properties more than electron temperature.

Abstract

Under high pressure, the group-VB transition metals vanadium (V) and niobium (Nb) exhibit simple crystal structures but complex physical behaviors, such as anomalous compression-induced softening and heating-induced hardening (CISHIH). Meanwhile, the impact of lattice thermal expansion-induced softening at elevated temperatures on HIH is yet to be investigated. Therefore, this study utilized ab initio (AIMD) and machine learning molecular dynamics (MLMD) to investigate the melting and abnormal mechanical softening-hardening behaviors of V and Nb under high pressure. Simulations reveal that the high-temperature Pnma phase of Nb reported in previous experimental studies is highly susceptible to mechanical instability and reverts to the body-centered cubic (BCC) phase. This discovery prompted a revised determination of the high-pressure melting line of Nb. The melting temperature of Nb significantly exceeds the existing theoretical and experimental estimate compared with that of V. AIMD simulations demonstrate that atomic thermal displacements have a greater influence on the HIH of V and Nb than pure electron temperature effects. In addition, the temperature-dependent anomalous elastic properties of V and Nb were investigated within a pressure range of 0-250 GPa using MLMD. The mechanical properties of V and Nb transitioned from HIH to heating-induced softening, elucidating the competition between thermal-expansion-induced softening and HIH. This study advances fundamental understanding of V and Nb physics, providing crucial theoretical foundations for establishing accurate equations of state and constitutive models for these metals.